Alpha modifier, or properties, that affect the child of the group "as a single object"
For Cycles, we can use &quot;Transparent depth&quot; and test if it is 1 or more and use it to drive the alpha value.
The disk below is a non transparent sphere behind the planes.

Less than means 1 if no transparency depth else 0.
Multiply that with your wanted alpha value and plug it to the alpha channel of the shader.

If you're using objects that have several layers (for instance replace the planes by Suzanne(s)), don't forget to tune the light path max bounces for transparency in the rendering settings (as if not, some transparent part may be rendered as solid):